1. Field of the Invention
The present invention relates to a method for enhancing the surface characteristics of metal surfaces, particularly metal coil composites and the decorative surfaces produced thereby.
2. Discussion of the Background
Various composite laminates are known wherein a metal sheet is laminated with a thermoplastic synthetic resin sheet. U.S. Pat. No. 4,508,425 discloses a mirror-like surface manufactured by plating chromium on one surface of a metal sheet bonded to a composite sheet, made up of a synthetic resin sheet and the metal sheet, to form the mirror surface. The mirrored finish sheet may be worked to a desired shape and may be formed with a decorative pattern.
U.S. Pat. No. 4,560,623 discloses a specular product of bronze-like tone particularly suitable for use as a decorative material. The specular product uses, as a substrate, a composite board comprising a synthetic resin sheet and metal sheets laminated thereon, and includes a nickel deposit plated on the metal sheet and a specular film of Snxe2x80x94Ni alloy electroplated on the nickel deposit using a specific electroplating bath.
Such composites are useful for a number of architectural applications, because the composites combine light weight with high strength. These composites may be used as finished surfaces for all or some portion of the interior or exterior surfaces of a building. However, the range of surface characteristics and patterns available in these patents is somewhat limited. Metal-resin composite laminates that mimic different wood grain, stone, and metallic surfaces for decorative or substitutive purposes are desired, but not readily available. The metal-resin composite laminates must also exhibit good weathering resistance with regard to temperature and humidity changes experienced during shipping and storage and capable of bending to a sharp angle without cracking of the coating on the exposed exterior surface of the metal. The composite must be capable of being cut to specified lengths, curved, routed, sawn, filed, drilled and fastened in order to complete fabrication of the desired item with the appropriate finish.
Thus, there remains a need for coated metal sheets that mimic different wood grain, stone and metallic surfaces, which exhibit excellent resistance to cracking, stress crazing, delamination, impact, and the like during fabrication of the item that requires the decorative surface. There also remains a need for proper methods to prepare such metal sheets and resultant composite laminates.
It has been known for sometime how to prepare colored articles from non-sealed anodized aluminum alloys in a form such as foil, sheet and containers by a thermal printing method as described in U.S. Pat. No. 4,177,299, incorporated herein by reference. More generally, methods have been described for decorative surface coatings of flat substrates using transfer sheets that are pressed on the substrate at elevated temperatures as included in U.S. Pat. No. 5,019,198. The pressing of the transfer sheet against the substrate is carried out with a press tool devoid of surface pattern and having a generally flat press surface.
Transfer printing such as sublimation printing transfers dyes or inks into the clear coating on the metal by pressing transfer paper against the clear coated surface under heat and pressure is well known. Transfer printing of inks or dyes into a variety of articles such as shirts, mugs, plastic articles and plastic coated substrates is also well known in the art. Dispersible dye crystals or inks are printed in reverse images in transfer paper or film that is to be used to transfer print the article to be decorated. The article to be decorated is usually made of plastic or has a plastic or polymer coating on it into which the dyes are transferred.
Most transfer printing is referred to as sublimation printing in which the dyes are said to sublimate under heat and pressure to be driven into any receptive substrate that is put in contact with the transfer paper. Such sublimation printing was developed circa 1969 and has been used extensively to print many articles including plastic coated metal substrates. Transfer printing also includes a melt printing process that is as described in several patents and patent applications including U.S. Pat. Nos. 4,587,155; 4,670,084; 4,668,239 and Published application WO 92/21514. According to U.S. Pat. No. 4,587,155, the desired dye image is transferred from the paper to the substrate by heating the dye to a temperature above its melting point but below its vaporization temperature so the dye will diffuse into the softened plastic substrate. Published Application WO 92/21,514 describes melt printing of planar metal base members such as aluminum, steel or the like which have been coated on at least one planar surface with a melt printable layer of softenable, dye-permeable thermoplastic or thermoset material such as polyethylene terephthalate, polybutylene terephthalate or other thermoplastic polyesters, polycarbonates, nylons and the like. Application WO 92/21,514 further describes bilayer coatings of thermoplastic or thermoset materials including a base coat optimally provided with a pigment and a second layer that can comprise a clear resin.
Canadian Patent 1,108,929 describes a process for applying designs to unscaled, anodized aluminum which is continuously heated to a temperature at which colored components of ink will sublime. The heated strip is fed into contact with an ink-carrying web so the ink is heated and transferred into the anodized aluminum surface. The decorated aluminum strip is then cooled with water and passed through a sealing bath filled with a sealing solution such as buffered aqueous nickel.
U.S. Pat. No. 5,643,659 details an intricate and elaborate heat transfer printing method for producing images having metallic luster, which would be visually similar to some examples of the present invention. This method describes a heat transfer printing sheet that produces the image by successively providing a transparent color layer with a coloring agent and hot-melt binder, and a metallic ink layer with metallic ink and hot-melt binder.
U.S. Pat. No. 4,352,721 describes a process for continuously printing an unsealed anodized aluminum strip by transfer from a carrier web of a design in printing ink containing colored sublimable components. This process includes feeding the aluminum strip to a heated moving surface allowing the carrier web to come in contact with the aluminum strip so that the printing ink is heated by contact with the anodized aluminum surface. In this manner the colored components of the ink transfer by sublimation from the carrier web to the aluminum strip. Sealing of the printed strip can then be accomplished either immediately after the printing of the unsealed surface or the printed strip may be coiled after cooling and subsequently sealed.
For a number of years, the coil coatings and industrial coatings industries have successfully coated unsealed, non-anodized and anodized metal substrates which have been anodized or etched by an electrochemical process employing sulfuric acid, chromic acid, phosphoric acid, or oxalic acid electrolytes. Such unsealed, non-anodized or anodized metal substrates provide an excellent base for adhesion of a paint, enamel or lacquer coating because of the surface porosity of the non-anodized or anodized metal surfaces.
Coating compositions for coating metal coils are often subjected to elevated temperatures during warehousing and shipping and especially during storage at coil coating plants where temperatures of about 120xc2x0 to 140xc2x0 F. (49xc2x0 to 60xc2x0 C.) may be encountered. These elevated temperature conditions can lead to delamination of laminated decorative coated compositions or cracking and peeling of painted surfaces for said compositions.
Transfer printing of metal substrates has heretofore frequently resulted in inconsistent and uneven transfer (mottling and/or ghosting) of the transferred images. Solid color areas have been difficult to produce in acceptable quality due to mottling. Ghosting is a faint offset second image that makes the product unacceptable. A process is therefore needed for producing a sharp and consistent image in a variety of mass-produced products including the aluminum composite building materials of the present invention.
Additionally, sealing of the anodized metal substrate can substantially reduce the abrasion resistance thereof. When anodized metal substrates have been sealed, it is very difficult for paint or a coating to adhere to the surface of the sealed, anodized metal substrate.
Furthermore, in producing unsealed, anodized metal parts, it has been necessary to first fabricate the desired part, then anodize the metal substrate. This extra step is time consuming and costly. A method is needed that eliminates the need for anodizing. It has not been previously possible to successfully improve the surface appearance of such metal substrates in coil form and thereafter fabricate satisfactory parts from the coated metal coil substrate.
Additionally metal substrate in coil form with the decorative finish must not become blocked, i.e. it is important that the coated coil is able to slide and unroll without sticking and binding.
Accordingly, one object of the present invention is to provide an improved process for transfer printing one or both sides of a metal substrate.
Another object of the present invention is to provide a transfer printed metal article, sheet or composite having one or more decorative surfaces having improved clarity and uniformity of print quality.
A further object of the present invention is to provide a decorative coating composition and process for a metal substrate or composite surface whereby the resulting product has sufficient integrity and adhesion to the metal surface, particularly coiled metal substrates.
A further object of the present invention is to provide a decorative coating for metal substrates or composites that allows post-coating fabrication of the coated metal into suitable parts of elements, particularly parts of elements for use with interior signage and displays, kiosks, building panels, etc.
Another object of the present invention is to provide a decorative coating that exhibits sufficient stability and integrity to permit the coated metal substrates to resist corrosion, hazing, cloudiness, flaking, cracking, mottling and/or delamination of the decorative coating when exposed to environmental factors such as smog, ultraviolet light, ozone, and excessive heat and cold, and abrasive cleaning solutions.
Another object of the present invention is to provide a decorative coating composition and process for an aluminum substrate or composite surface that avoids the need for anodizing.
It is another object of the present invention to provide a method for preparing a decoratively coated metal plate or composite, which may be bent to a sharp angle without cracking of the coating.
These and other objects and advantages of the present invention have been satisfied by the discovery of a method for transfer printing of a metal article by coating a surface of the metal article with an adhesive primer followed by a layer of a polymer thermoset resin, curing the thermoset resin, then applying a printed dye image to the resulting layer of cured polymer thermoset resin by use of a transfer film comprising a carrier film having a reverse printed dye image coated on one side of the carrier film, by applying pressure and heat to the transfer film, then removing the carrier film, leaving the printed dye image imprinted on the cured polymer thermoset resin, and the resulting coated metal articles obtained thereby.